Magnetic resonance imaging (MRI) is currently contra-indicated for patients who have implanted medical devices. This is due largely to the patient safety issue that results when the strong electromagnetic fields of an MRI system interact with the antenna-like therapy delivery leads of an active implantable medical device (AIMD). It is well documented that the radio frequency (RF) signals that are generated by the MRI system can couple along the length of a lead body and create induced current loops. These current loops can cause significant heating at points of high current concentration, the most significant of which is the distal tip, where the lead system makes direct contact with body tissue.
As disclosed in U.S. patent application Ser. No. 11/558,349, filed Nov. 9, 2006, TANK FILTERS PLACED IN SERIES WITH THE LEAD WIRES OR CIRCUITS OF ACTIVE MEDICAL DEVICES TO ENHANCE MRI COMPATIBILITY, and Provisional Patent Application No. 60/968,662, filed Aug. 29, 2007, entitled A CYLINDRICAL BAND STOP FILTER FOR M LEAD SYSTEMS, the contents of which are incorporated herein by reference, a novel method to minimize the expected heating at the distal tip of the lead system is to incorporate a bandstop filter. This bandstop filter is comprised of an inductor and capacitor in parallel, with the entire filter connected in series to the lead system. In such a system, the bandstop filter can be constructed so that its resonant frequency or frequencies coincides with the RF operating frequency of one or more MRI systems.
RF frequencies are directly related to the MRI machine static magnetic field by the Lamour Relationship. Typical values are 64 MHz for 1.5 T systems, and 128 MHz for 3.0 T systems. At resonance, the impedance of the bandstop filter is quite high which reduces the flow of current at the MRI RF pulsed frequency thereby reducing leadwire and/or electrode heating. Increasing the impedance at the distal tip also greatly reduces the amount of RF current that would flow into body tissue. It has been documented that excess current can cause tissue damage or even tissue necrosis.
Implementation of this technology in implantable leads is a significant challenge. Bandstop filters for use in lead systems must be biocompatible, not significantly change the electrical performance characteristics of the lead (except within the context of the invention), and must not significantly affect size, weight, or implantability. With increasingly smaller leads being developed to accommodate small vasculature and left ventricular pacing through the coronary sinus, bandstop technology must be equally scalable to match the same demands.
The present invention satisfies these needs and provides other related advantages.